I accidentally drove my 2000 Grand Marquis on second gear on the highway. I was going 65 for like 5 minutes and tried to get it to 70mph but the car was shaking, when I realized I put it into regular drive while going 65 and it stopped shaking.

4 Answers
4

It's definitely not great. The redline is there for a reason. That shaking was likely the engine speed limiter looking at the RPMs and saying, "yes, that's high enough." The speed limiter can be either an ignition cutoff or a fuel cutoff - both will make the car feel very shaky.

The most common bad consequence of an overspeed condition when the engine speed limiter fails to thwart the operator is a bent valve. There's a lot going on in the engine when it's turning at several thousand RPMs: if the valve does not have time to return to its seat before the piston comes back up, the piston is going to hit the valve harder than you would like.

Of course, that's not the only possibility, just the most common that I've heard of.

In short, listen to your car and be glad that the limiter worked that time.

Hmmmm. Pretty sure that's what happened to my Corolla a few years ago. Never really knew what happened to it, but I was messing around on ice and the revs got pretty high. Makes sense.
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Vian EsterhuizenMay 9 '11 at 20:46

@Vian, the worst part about the redline is that it isn't a sure thing. It's not a contract from the vendor that says "below this line good, above this line BOOM!" Instead it's really just a statistical measure. At the redline, the probability that something bad will happen is much greater. Higher than that, it's higher. However, you could have a bad day, touch the redline and bend something.
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Bob Cross♦May 9 '11 at 21:49

Yeah. It was just personal commentary. I'm about 99% sure that I bent a valve. That makes a lot of sense for what happened to me.
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Vian EsterhuizenMay 9 '11 at 21:51

You probably did no immediate damage. Most cars have an electronic limiter that keep the engine RPM from going into the redline, which could cause a blown engine. However, if you kept doing that, your engine would wear out a lot faster.

More details of the vehicle in question would be helpful to be sure. However, from personal experience, my guess would be that the shakes are not solely due to the engine being run at high RPM.

The Mercury Grand Marquis is equipped with a 4.6L Modular V8 engine and 4R70W automatic transmission, similar to many other products of the Ford Motor Company. Notable among these, at one point or another, have been the Ford Mustang, Ford Crown Victoria, Lincoln Town Car, and the Lincoln Mark VIII.

In the Lincoln Mark VIII, 65-70 MPH in 2nd gear was very doable. In fact, redline (6,000 RPM) for 2nd gear got you going at around 90 MPH. Maintaining such a high engine speed for long durations would definitely get things heated up a bit, but it's nothing the car can't handle as long as you don't do it for too long or too often.

I once had to do this myself to take the Mark VIII for repairs after the one-way clutch had failed - rendering the transmission unable to engage any gear higher than 2nd. To get to the shop where the work was going to be done required about a half-hour or so of highway travel. Parts of the highway were 65-70 MPH zones, where of course most drivers were running 75-80+. The way I managed the trip was to take the car up to speed until the engine started to get too warm, and then coast in neutral long enough to let it cool down to a more regular temperature before engaging the transmission for another run. I definitely don't recommend doing this if it can be avoided, but it should serve as a case example for two points:

It's doubtful that your car was redlining at 65-70 MPH in 2nd gear.

A one-time incident of approximately 5 minutes' duration at that speed, while not exactly good for the car, should not cause significant damage to an otherwise healthy drivetrain.

Given the shaking that you've experienced though, I'd definitely want to have a professional take a look at the car.

most newer cars automatics shift gears for you so as not to let you blow the engine. They put this in so people who bought a new car would stop blowing the engine and saying i dont no what happen (tring not to have to pay for the car lemon)put your car or truck in low gear gun it going down the road it will shift for you.